A remotely installable jet pump diffuser weld repair device structurally replaces cracked adapter or lower ring to tail pipe welds and the tail pipe to shell welds. The device is remotely installable in the limited space in the shroud to vessel annulus of boiling water reactor (BWR) power plants.
The jet pump assembly is part of the reactor recirculation system. Each assembly includes a riser assembly, a riser brace, two inlet-mixer assemblies, and two diffuser assemblies. Each assembly is installed in the annulus between the reactor pressure vessel (RPV) and the shroud. There are twenty jet pumps (ten jet pump assemblies) installed in a typical General Electric BWR. The riser assembly is welded to the reactor pressure vessel (RPV) at the riser brace location, and the recirculation inlet nozzle at the penetration. The two diffuser assemblies (DA) are welded to the shroud support plate (see FIG. 1).
The two inlet-mixer assemblies are removable components. The entrance end of each inlet-mixer assembly seats into the top of the riser transition piece. The exit end fits into a slip joint with the top of the diffuser assembly. Lateral support for the inlet-mixer is provided by the restrainer bracket, which is welded to the riser pipe. Two adjusting screws (set screws), each threaded into the restrainer bracket, and the inlet-mixer wedge provide three points of lateral support for the inlet-mixer. Lateral support for the riser assembly is provided by the riser brace.
Existing jet pump assembly components are fabricated from Type 304 stainless steel, with the exception of the diffuser lower ring, which is fabricated from Ni—Cr—Fe Alloy 600. The welds are designated DF-3 or DF-2 as shown in FIG. 1. Cracking attributed to Intergranular Stress Corrosion Cracking (IGSCC) has been observed at the Alloy 600 to stainless steel transition weld (DF-3) between the lower ring and tail pipe.
The postulated crack in a DF-3 or a DF-2 weld would detach the diffuser from the jet pump assembly, resulting in the total loss of the jet pump assembly. This could in turn result in failure of jet pump functionality in providing recirculation flow path to reactor core and loss of providing the ⅔-core height coverage in the event of LOCA accident. The failure of a jet pump assembly occurring during operation will cause a change in the monitored jet pump flow, which would be detected, allowing the plant to be brought to a safe shut down condition in accordance with plant technical specifications.
Jet pump diffuser weld cracks that have required repair have been a relatively recent occurrence. There has only been one other type of jet pump diffuser repair developed using a tie rod assembly. The scope of that repair was limited to hardware design and analysis and installation tooling concept design. Full scale mockup testing was not done. Repair clamps for welds in other tubular portions of the jet pump have been developed specifically for the thermal sleeve to elbow weld in the riser portion of the jet pump. U.S. Pat. Nos. 6,053,652, 6,108,391 and 6,086,120 relate to jet pump riser thermal sleeve to elbow weld repair.